The present invention relates to a method for verification of at least one given fuel mixture ratio in a fuel injection system during operation of an internal combustion engine with at least two separately stored fuel types, wherein the injection system is supplied with at least one first fuel type.
The use of vegetable oils as biomass fuels offers ecological and economical advantages as compared with fossil fuels. Vegetable oils are obtained from renewable raw materials, which mean that during combustion they are essentially carbon dioxide neutral, free of sulfur and, as opposed to conventional fuels, biologically degradable. Furthermore, the discharge of particles during combustion is reduced drastically in comparison with diesel.
Due to the different chemical and physical fuel properties of vegetable oils as compared with diesel fuels or bio diesel, specially developed vegetable oil engines or modified series production diesel engines must be used in order to operate standard automobiles with vegetable oil. However, such a use of natural vegetable oil as a fuel causes considerable technical difficulties especially in modern series production direct fuel injection diesel engines.
Conventional fuel injection systems of nearly all standard series production diesel engines feature for example one fuel line or supply line from a fuel tank, in particular a diesel tank, to an injection system and one return line from the injection system back to the fuel tank. During operation for example of a series production diesel engine the volume flow in the circulation from the fuel tank through the injection system and back to the fuel tank is on average a multiple of the actual quantity of fuel used by the series production diesel engine per time unit. The fuel circulating thus in the injection system serves to lubricate the mechanical parts and also transports the operating heat out of the injection system. In many fuel injection systems the returning fuel is therefore used additionally to heat a fuel pre-filter, for example.
In new types of fuel injection systems with a radial piston pump and especially in systems with the “common rail” or “pump-nozzle” or “pump-line-nozzle” technology, on the other hand, the fuel circulating in the injection system is used especially for cooling. Due to mechanical work and the configuration of the components supplying the fuel, operation of such a fuel injection system results by principle in a thermal power of several kilowatts, which is compensated by means of the fuel circulation. Before returning to the fuel tank, the heated fuel frequently passes through a heat exchanger or return condenser. Such a return condenser maintains thermal stability in the injection system. Furthermore, gas bubbles that have formed in the fuel or inadvertently admitted air are continuously removed from the injection system by means of the return line.
For the use of fuels with more favorable properties for the injection and combustion in the engine at higher fuel temperatures, it has proven useful to bypass the fuel tank—wholly or partially—during the return of the fuel to the forward flow (short-circuit return). This process is used to influence the flow properties of fuels with highly temperature-dependent viscosity and can additionally be based on the gradient of the bubble-point curve, the ignition temperature or other chemical or physical properties. DE 197 36 283 describes a fuel supply system in which a return line to the tank is provided to protect the fuel from overheating based on its viscosity.
Furthermore, methods for operating an internal combustion engine with two separately stored fuel types, for example with diesel fuel and vegetable oil, are known, in which a diesel engine is supplied with either diesel fuel or heated vegetable oil depending on the temperature of the engine's cooling water. Such systems are referred to in the literature as 2-tank systems. DE 202 08 590, for example, describes a 2-tank system in which the fuel is heated by means of a cooling water-operated heat exchanger and an electric heating source, wherein the vegetable oil is preferably heated to 70° C. DE 38 00 585 likewise describes a temperature-dependent switching of the fuel supply for operating an internal combustion engine with two separate fuel types.
The now widespread direct fuel injection diesel engines have significantly higher operating requirements for the complete combustion of the injected fuel than is the case with pre-combustion chamber or whirl chamber diesel engines. In addition, the applicable and future emissions standards are a decisive factor in the design of new diesel engines and corresponding injection systems. Due to the current emissions standards it is necessary to keep the nitrogen oxide and soot emission levels of newly developed diesel engines as low as possible.
For this purpose, exhaust return flow systems are known in the art and are installed in a multitude of commercially available diesel vehicles. In such exhaust return flow systems a lower oxygen concentration is provided for the combustion process by mixing exhaust gas with the intake air. This avoids local peak temperatures during the combustion process, which contribute to the thermal formation of NO (Zeldovich NO). The goal of preventing nitrogen oxide is in direct conflict with the formation of soot, since the soot is not sufficiently oxidized at a low oxygen concentration.
A further measure for reducing the nitrogen oxide emissions is the controlled dynamic adjustment of the injection (at times also several injections per cycle) or of the start of supply to “retard”, i.e. at a position of the piston before or after top dead center that is reached at a later point in time. The exhaust behavior can also be affected by the injection pressure. A lower injection pressure causes the formation of larger droplets, which in turn results in a higher soot content.
The technical design and the regulation of a fuel injection system are optimized for the use of regular diesel fuel in modern direction injection diesel engines. Exhaust return flow, retard, etc. are achieved for example by an electronic control during operation of the vehicle based on the current operating parameters and comparison values that are optimized for diesel operation. Now, if pure vegetable oil is used instead of diesel fuel, for example, such measures can frequently cause malfunctions or serious engine damage.
Current tests of corresponding conversion systems for the operation of series production diesel engines with vegetable oil have shown that serious problems occur especially with direct injection diesel engines, in particular with “common rail” and also “pump-nozzle” diesel engines. For example, the complete combustion of the vegetable oil is frequently not ensured under certain operating conditions, in particular during a cold start or when the diesel engine is operated under a moderate load. Such unfavorable operating conditions occur regularly and frequently especially with automobiles, commercial vehicles and agricultural machines. Possible consequences of incomplete combustion are the entry of unburned vegetable oil in the motor oil, resulting in the formation of clumps due to polymerization. This causes a breakdown in the lubrication system. In addition, changes in the injection nozzles (“coking”) cause a defective injection pattern, which can result in serious damage to the piston heads, the piston rings and the valves.
Without internal modification to the engine, i.e. only with changes in the peripheral units of the engine and in the fuel supply system, such direct injection series production diesel engines cannot be converted for operation with pure vegetable oil with any appreciable degree of reliability, long-term stability and convenience, also taking into account regularly occurring unfavorable operating conditions.
Both for the cold start and the warm-up phase of such a diesel engine and during certain operating conditions, it has proven technically advantageous and desirable to supply the fuel injection system with pure diesel fuel, pure vegetable oil or a mixture of vegetable oil and diesel fuel. Even a small percentage of diesel fuel in a mixture with vegetable oil significantly reduces the viscosity of the diesel-vegetable oil mixture as compared with pure vegetable oil and generally also distinctly improves combustion in the internal combustion engine. For this purpose, a defined mixture ratio of the fuel mixture located in the fuel injection system based on the respective operating condition of the diesel engine is necessary.
Particularly during the starting process of a diesel engine it is necessary that the injected fuel be immediately ignitable and completely combustible. Especially during a cold start after being parked for an extended period and after cooling of the diesel engine accordingly, the fuel in the injection system should be replaced wholly or at least partially with diesel fuel. This process is referred to as “flushing” and preferably is also carried out before shutting off the engine, while the engine is still running. For this purpose, the diesel engine is supplied only with diesel fuel for a specified duration, which continuously increases the proportion of diesel in the fuel mixture in the injection system. Due to the design of the injection systems, flushing is essentially a dilution process and only to a small degree a displacement process. The proportion of vegetable oil in the overall quantity of fuel within the injection system during the flushing process can therefore be described in good approximation based on an exponential decay curve, which depends only on the supplied quantity of diesel fuel for a given injection system.
If the fuel or the fuel mixture is supplied as described above in a short-circuit return flow, the supplied quantity of diesel fuel corresponds exactly to the quantity of fuel injected into the combustion chambers. Knowledge of the consumption of the vehicle makes it possible to estimate the diesel concentration based on the distance traveled or the time that elapses after initiation of the flushing process. Typical distances for a “sufficient” flushing process are 5-10 km before shutting off the engine.
For the purpose of supplying diesel fuel more quickly, the flushing process can be implemented so that a return flow to the tank is ensured. Then the quantity of diesel supplied is determined not only based on consumption, but additionally by the quantity of fuel returning to the fuel tank. The latter is by far the dominating factor in this process.
A measurement of the time or distance after initiation of the flushing process does not allow for reliable information on the current mixture ratio of the fuel mixture in the injection system under otherwise non-constant external conditions.
The volume flow into the fuel tank corresponds to the volume pumped by the injection system and depends on a number of parameters. The exact determination of these parameters is not directly possible. At best, average values can be estimated over extended periods, by averaging the relevant parameters. A typical magnitude for the volume flow through a distributor injection pump during operation is for example approximately 60 liters per second. Therefore, a flushing process under suitable ambient parameters can be completed in less than one minute.
The exact determination of the percentage mixture ratio of the fuel types is not always necessary for this purpose. However, it is particularly important to be able to reliably determine a point in time for the flushing process at which a pre-determined mixture ratio has been reached during operation of the diesel engine, after the supply of fuel has been switched from one type to another. In addition, it is necessary to be able to set a selectable mixture ratio of the fuel types during operation and to be able to automatically maintain the set mixture ratio, without resulting in mixing of the fuels in the fuel tanks.
It is an object of the present invention therefore is to provide a method for verification of the presence of at least one given fuel mixture ratio, in order to enable, during operation of an internal combustion engine, the reliable verification of a given fuel mixture ratio and the setting and maintaining of this fuel mixture ratio and the determination of the load condition of an internal combustion engine.